In various fields such as a semiconductor manufacturing process, liquid crystal manufacturing process, and precision machining, vibration control for blocking or suppressing disturbance micro vibration is widely used. For each of microfabrication and inspection apparatuses used in such processes, such as scanning electron microscopes and semiconductor exposure apparatuses (steppers), strict conditions of acceptable vibration for ensuring the performance of that apparatus are required. Also, an active-type precision vibration isolation table that uses actuators to support an apparatus susceptible to disturbance vibration as well as controlling the actuators so as to diminish the vibration has been used.
In an active vibration isolation table, which is a pneumatic servo apparatus using pneumatic actuators, in order to control the pressure and flow rate of gas in each of the actuators, a nozzle flapper valve has been used. FIG. 49 is a structural diagram modelling a working principle common to conventional servo valves. The structure of a servo valve can be roughly divided into an actuator part A-1 and a fluid control part B-1. In the actuator part A-1, reference character 551 denotes a magnet (permanent magnet), 552 a coil, 553 a body containing the coil, 554 a flapper, 555a and 555b a pair of yokes attached with fore ends thereof facing toward each other, 556 a flapper fore end part on the actuator side, 557 a leaf spring also serving as a seal member, and 558 a support center part for the leaf spring.
In the fluid control part B-1, reference character 560 denotes a forward nozzle, 561 a reverse nozzle, 562 a flapper fore end part on the fluid control part side, 563 a supply port, 564 an exhaust port, 565 a load port (control port), and 566 a control chamber.
A gas having a supply pressure Ps is supplied to the control chamber 566 via the forward nozzle 560. At the same time, the gas inside the control chamber 566 is flowed out into the air via the reverse nozzles 561. The difference between an inflow amount from the forward nozzle and an outflow amount from the reverse nozzle determines a control pressure Pa inside the control chamber 566 and an outflow amount from the load port 565. Note that the structure of a servo valve actually used is a three-dimensional structure in which a magnetic circuit based on a permanent magnet and a magnetic circuit based on an electromagnet are orthogonally arranged. The above-described basic structure of the pneumatic servo valve has been created secondarily as a result of applying a long-established hydraulic servo valve technology, and is used as that of a primary control valve (pilot valve) of an electrohydraulic control valve.